1. Field of the Invention
The present invention generally relates to the field of write drivers for operating inductive recording heads. In particular it relates to a technique for increasing the speed of write drivers which use an H configuration.
2. Background Description
A disk drive has inductive write heads that write information to the disk when current is switched through them. The switching current creates a magnetic field that is recorded on the disk medium. It is desirable to switch the head current from a positive value to a negative value (and vice versa) very fast so that data bits can be packed closer together on the disk for a given rotational speed. This leads to a higher density on the disk and allows a high data transfer rate.
At one time conventional write drivers consisted of resistively-loaded emitter-coupled pairs of bipolar transistors. The transistors served to switch a constant current through the write head, which was connected between the collectors. By alternatively turning on the switches, bidirectional current pulses were generated in the head. This approach was not particularly fast and dissipated a substantial amount of power because the circuit required twice the amount of current that was delivered to the head.
In more recent times, it has become standard to switch current in the write head using four transistor switches in an H configuration. This traditional circuit for switching the current in the write head is called an H-driver because the circuit, schematically, is shaped like an "H." The usual H-driver configuration includes upper write switching or "pull-up" transistors and lower "current-switching" transistors. The pull-up transistors are connected between a positive supply voltage and the write head contacts. The current switching transistors are connected between the write head contacts and a constant current sink. Bidirectional current pulses are generated by alternately turning on diagonally opposed switches which steers current in a positive and negative direction through the head. This circuit is faster than the older conventional design and dissipates only half the power.
Two examples of prior art circuits are shown in FIG. 1. The inductive write head is represented with the box called "load." In FIG. 1a the bottom current sources are switched so that the flow of current is either S1-load-S4 or S2-load-S3. Thus, the switching action changes the polarity in the load. FIG. 1b shows a similar arrangement, but the current source itself is not switched; rather, there is a pair of switches that direct which side of the circuit the current flows in.
However, as the state of the art advances there is a need for faster drive circuits which operate with lower supply voltages. These two objectives are in conflict because switching speed through an inductive load is proportional to the voltage swing at the load; furthermore, not all the supply voltage is available to the inductive load because there are voltage drops across the "pull-up" switches, current switches, and current source. Consequently, a variety of techniques have been developed for increasing the voltage swing at the write head load and limiting voltage drops elsewhere in the circuit. For example, in U.S. Pat. No. 5,331,479 to Madsen there is described a capacitive charging circuit for increasing the voltage across the write head. Another capacitive charging circuit is described in U.S. Pat. No. 4,647,988 in connection with a three terminal inductive write head topology. Current mirrors having the effect of reducing voltage drops elsewhere in the H-driver circuit and improving voltage swings across the write head are described in U.S. Pat. No. 5,386,328 to Chiou et al. and in U.S. Pat. No. 5,287,231 to Shier et al.
The present trend in disk drive technologies is to fly the write heads closer to the disk surface for increased coupling efficiency. This, and the fact that head materials and disk platter materials are continuously being improved, has lead to a drop in the inductance of the heads and the write current needed to write data to the disk. Many years ago the inductance of a write head was typically several microhenries whereas now it is typically less than 100 nanohenries. This reduction in inductance, and the increase in switching speed, has lead to a change in the factors that control the current rise time in the head. Older generations of write drivers and heads were mostly limited by the inductance and voltage that could be induced across the head (compliance voltage). The current rise time was proportional to the head inductance divided by the voltage. The switches were able to switch much faster than this and thus their speed did not affect the overall rise time of the current in the head. However, in today's technologies, the switching time of the transistor switches themselves are of the same order of magnitude as the voltage/inductance limited times. The switching times of the devices can actually dominate the overall current switching event. In a traditional H-driver it is imperative that the switching action of both the top and bottom halves of the H-driver occur simultaneously in order to achieve the fastest reversal of current. Any mismatch in the top and bottom switching events leads to longer switching times for the current in the head.
Mismatches may occur because the times required to accomplish the switch are different between the top and bottom halves of the H-circuit. Mismatches may also occur because switching action does not begin at the same time. Process variations tend to aggravate these problems, especially if the top devices are different than the bottom devices and they do not track each other, as is often the case in circuits available from practical manufacturing processes. Even if the switching times are perfectly aligned, in terms of the starting of the transitions, if one of the transitions (either top or bottom) takes longer than the other, the total output current reversal time will be governed by the slowest switching event. These switching events are very difficult to control in practical manufacturing processes with the sub-nanosecond precision required for today's fast drivers.
Various circuits have been used in the prior art to minimize timing mismatches in write head drivers. U.S. Pat. No. 5,333,081 to Mitsui teaches use of a delay circuit which introduces mismatches to avoid overvoltage breakdown, which can actually increase switching time.